1. Field of the Invention
This invention relates to T-coil structures and fabrication methods, and their use in automatic test equipment (ATE).
2. Description of the Related Art
ATE systems employ drive channels for applying test signals to a device under test (DUT), and comparator circuits for receiving signals back from the DUT and comparing them to a threshold to determine the DUT's response. Drive circuits can include voltage mode drivers, in which a dynamically varied voltage signal is generated directly for application to the DUT, current mode drivers in which a dynamically varied current is generated and directed through a resistor to a constant voltage reference to generate a dynamically varying voltage signal on the other side of the resistor for application to the DUT, and combinations of the two. An example of a combination of both types of drivers is provided in U.S. Pat. No. 6,292,010.
Such circuits have associated capacitances that reduce their bandwidth and speed. Contributors to the overall capacitance include collector-base and collector-substrate capacitances of the output transistors in current mode drivers, the capacitance of the cable that connects the circuits to the DUT, and capacitances associated with metal runs and bond pads on the comparator circuit.
Prior attempts to eliminate or compensate for these capacitances have included designing class AB drivers to be faster than necessary, and then adding filters to compensate for the driver's capacitance. This makes the driver unusable for the upper end of its design speed, and also lowers the performance of the comparator. Separate cables have also been provided for transmitting the drive signal to the DUT and directing the DUT's response at pin to the comparators. This requires an additional cable for each drive channel, and also requires the driver circuitry and comparators to be provided on separate chips. Considering that typical ATE systems can have hundreds of drive channels, the additional expense and space required can be significant.
An “inductive peaking” technique has also been moved used, in which the driver output bond pad was moved so as to increase the bond wire length and thereby increase the circuit's effective inductance. This at least partially compensated parasitic capacitance to boost the circuit output. However, its compensation effect was limited, since it provided compensation only for transmitted drive signals but not for DUT response signals.